1. Technical Field
The present invention relates to a wireless communication system comprised of a base station and mobile stations, particularly to an improvement relating to inter-mobile station direct communication in a wireless communication system based on a TDMA (Time Division Multiple Access) system.
2. Background Art
A wireless communication system is comprised of a base station for giving notices of allocations of bands (time regions for communications) and a plurality of mobile stations based on the TDMA system (in which different time regions are allocated to the mobile stations at the same frequency for carrying out communication). For such a system, a PHS inter-mobile station direct communication system in accordance with RCR STD-28 (second-generation cordless telephone system standard, version 1) of ARIB (Association of Radio Industries and Businesses) has been proposed as a method of carrying out inter-mobile station direct communication at a frequency other than that used by the base station. Improvements of the system are disclosed in JP Patent Publication (Kokai) Nos. 11-122663 A1(1999) and 11-341564 A1(1999), for example.
FIG. 13 shows an example of the frame structure in the TDMA wireless communication system.
As shown in FIG. 13(a), a length of time in which the same frequency is used for communication is divided into frames 1 at predetermined time intervals. In each frame (at predetermined periods), the base station transmits control information 2 to all of the mobile stations in a wireless LAN area created by the base station. The control information 2 includes information allowing the multiple mobile stations to be synchronized with the base station, and information indicating the band (time region) within the frame allocated to each mobile station.
The frame is divided into the time regions of DOWN-LINK 3 for the transmission of data from the base station to the mobile stations and UP-LINK 4 for the transmission of data from the mobile stations to the base station. In each time region, each mobile station is allocated a band 5 (band A) and a band 6 (band B), as shown in FIG. 13(b), such that the base station and the mobile stations can communicate with one another bi-directionally.
The control information 2 will be hereafter described by referring to FIG. 13(c).
Generally, in the physical layer of wireless systems, a digital signal comprised of a preamble 200 for broadcast and a data payload 201 (data payload 1, . . . , data payload N), which is information data for transmission, is converted into an electric signal. The preamble 200 located at the head identifies the signal received from a wireless interface. Particularly, the preamble 200 for broadcast that is attached when the base station transmits broadcast information into a wireless cell functions as a synchronization signal with which a mobile station in the wireless cell attempts to achieve synchronization with the base station. Further, the control information 2 includes such control information as frame structure information and band allocation information.
FIG. 14 shows a block diagram of an example of the configuration of a wireless LAN based on the above-described wireless communication system.
In FIG. 14, numeral 7 designates a WAN (Wide Area Network), and numeral 8 designates a wireless LAN (Local Area Network). The wireless LAN 8 is comprised of a plurality of mobile stations 11-13 and a base station 10. The base station 10 is either connected to a central control unit 9 for the central control of band allocations, or is equipped with the central control unit 9 inside. The wireless LAN 8 is connected to the WAN 7 via the base station 10.
The mobile stations 11 to 13 carry out a base station-mobile station communication according to the band allocation information notified by the base station 10. The communication paths are indicated as communication paths 14-16. The communication path for direct communication between a mobile station 11 (mobile station <1>) and a mobile station 12 (mobile station <2>) is indicated by a communication path 17.
When the mobile station 11 (mobile station <1>) transmits data to the mobile station 12 (mobile station <2>), if the normal base station-mobile station communication is employed, the same data would have to be transmitted twice via the communication paths 14 and 15 shown in FIG. 14, which is inefficient. Thus, it is necessary to use a method for carrying out inter-mobile station direct communication by which data is directly transmitted between the mobile stations via the communication path 17 without the intervention of the base station 10.
FIG. 15 shows an example of the frame structure for inter-mobile station direct communication in a conventional wireless communication system.
In the example of FIG. 15, inter-mobile station direct communication is conducted at a frequency different from the one used by the base station 10. One of the mobile stations that carry out inter-mobile station direct communication transmits control information 19 at the frequency for inter-mobile station direct communication. This frequency is different from the frequency at which frame synchronization is achieved and communications are carried out according to the control information 18 transmitted by the base station. The mobile stations that carry out the inter-mobile station direct communication are provided with a base-station function for synchronizing their frames and allocating bands.
Other examples of conventional techniques for carrying out inter-mobile station direct communication include: a wireless LAN system and a PDC (Personal Digital Cellular) portable telephone employing the CSMA/CA (Carrier Sense Multiple Access with Collision Avoidance) system according to the ISO/IEC (International Organization for Standardization/International Electrotechnical Commission) 8802-11 or the IEEE (Institute of Electrical and Electronics Engineers) 802.11 standards; HiperLAN according to the ETSI (European Telecommunications Standards Institute), which is a 3.5-generation system transitioning into a 4th-generation system to be standardized around the year 2010 following PHS (Personal Handy-phone System) and the W-CDMA (Wideband Code Division Multiple Access); and the MMAC (Multimedia Mobile Access Communication System) wireless system according to the ARIB (Association of Radio Industries and Businesses).
The IEEE802.11 and the HiperLAN will be briefly described below, although they do not have direct relevance to the present invention in that the former is not a centrally controlled wireless system comprising a central control unit for centrally controlling band allocations, and that the latter is a system that employs the same frequency as that for base station-mobile station communication.
FIG. 16 shows an example of communication according to the IEEE802.11 system.
In FIG. 16, among mobile stations 1 to 4 in a wireless LAN area 20, when a mobile station <1> wishes to send information to a mobile station <2>, the mobile station <1> broadcasts an RTS (Request to Send) signal 21 and 24. Each mobile station analyzes the RTS signal. When it is not an information transmission request directed to a particular mobile station, the mobile station stands by. When the RTS signal is directed to a particular mobile station, the mobile station (mobile station <2>) transmits a CTS (Clear to Send) signal 22 and 25 to indicate that it is ready to receive. In response, the mobile station <1> transmits information via a MAC signal 23 and 26 to the mobile station <2>. During these processes, the other mobile stations refrain from transmission for a certain time period to avoid collision of transmission signals.
FIG. 17 shows an example of the frame structure in the HiperLAN system. Parts or elements similar to those shown in FIG. 13 are designated by similar references.
In the HiperLAN system, a band 28 is provided in a TDMA frame 27 for carrying out inter-mobile station direct communication. In this band 28, the base station ceases transmission to allow the mobile stations to transmit to each other, thus allowing base station-mobile station communication and inter-mobile station direct communication to take place on a single frequency.
In a conventional wireless communication system, the frames for inter-mobile station direct communication and those for base station-mobile station communication are not synchronized, as shown in FIG. 15. Thus, in order to obtain control information for synchronizing the respective frames, the communication modes are switched. This is a process in which a mobile station terminates its connection with the base station for base station-mobile station communication and then sets up a connection with the mobile station that has the base-station function for carrying out inter-mobile station direct communication. An improved method of obtaining broadcast information from the base station is proposed by JP Patent Publication (Kokai) No. 11-122663 A1(1999), for example, in which the switching is carried out intermittently.
However, the apparatus disclosed in the above publication has the problem that the mobile station 11 (mobile station <1>) cannot carry out data communication with the mobile station 12 (mobile station <2>) and the base station 10 simultaneously, as shown in FIG. 14. In recent communication systems, there is an ongoing shift from voice communication by telephone to data communication. Thus, the above problem means that, in the context of building a wireless LAN, the mobile stations are intermittently cut off from the network, creating further problems. For example, the base station may not be able to transmit data received from a connected WAN (such as the Internet) to a destination mobile station in the wireless LAN, or the base station may not even recognize the presence of the mobile station due to the absence of connection therewith.
The above discussion is based on the assumption of creating, for example, a household wireless LAN (home network) comprised of a gateway unit including a base station connected to the Internet outside, and mobile stations including household information appliances (such as a refrigerator, microwave oven, television, video server, or set-top box, for example). In this case, it would be problematic if the external network could not send video information to the video server when the video server is wirelessly transmitting video information to the television, or if, when an air conditioner should be externally operated, the presence of the air conditioner on the network could not be confirmed due to the termination of its connection to the base station.
Further, in order to realize inter-mobile station direct communication, there is the additional problem of having to add the base-station function to one of the mobile stations that is to act as the base station.
It is an object of the invention to provide an efficient wireless communication system that allows for base station-mobile station communication even when inter-mobile station direct communication is carried out at a frequency other than that of the base station, which can reduce the burden on the mobile stations by having the base station control inter-mobile station direct communication, and which allows a plurality of frequencies to be simultaneously used by a single base station.